1. Field of the Invention
The disclosed invention relates to a semiconductor device using a semiconductor element and a method for driving the semiconductor device.
2. Description of the Related Art
Memory devices using semiconductor elements are broadly classified into two categories: a volatile memory device that loses stored data when power supply stops, and a nonvolatile memory device that holds stored data even when power is not supplied.
A typical example of a volatile memory device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a memory element is selected and electric charge is stored in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost according to the above-described principle; thus, another writing operation is necessary every time data is read out. Moreover, a transistor included in a memory element has leakage current (off-state current) between a source and a drain in an off state or the like and electric charge flows into or out of the transistor even if the transistor is not selected, which makes a data holding period short. For that reason, a writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, another memory device utilizing a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile memory device is a static random access memory (SRAM). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need a refresh operation, which is an advantage over a DRAM. However, cost per storage capacity is higher because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a nonvolatile memory device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that the data holding time is extremely long (almost permanent) and a refresh operation which is necessary to a volatile memory device is not needed (e.g., see Patent Document 1).
However, in a flash memory, there is a problem in that a memory element becomes unable to function after a predetermined number of writing operations because a gate insulating layer included in the memory element deteriorates due to tunneling current generated in writing operations. In order to reduce effects of this problem, a method in which the number of writing operations is equalized among memory elements can be employed, for example, but a complex peripheral circuit is needed to realize this method. Moreover, even when such a method is employed, the fundamental problem of lifetime is not resolved. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary in order to inject electric charge into the floating gate or remove the electric charge, and a circuit therefor is required. Further, it takes a relatively long time to inject or remove electric charge, and it is not easy to increase the speed of writing and erasing data.
Another example of a nonvolatile memory device is a magnetoresistive random access memory (MRAM) which is a memory device including a magnetic material. An MRAM consumes a comparatively large amount of current in a writing operation; therefore, there is a problem in that it is difficult for an MRAM to concurrently perform a writing operation in a number of memory cells.